1. Field of the Invention
The present invention relates to a focus adjustment apparatus and an image capturing apparatus, which automatically adjust a focus based on a video signal from an image sensor.
2. Description of the Related Art
Recently, a digital single-lens reflex camera with a function of displaying an image of a subject in an electronic view finder or on a display panel, so-called a Live View (hereinafter, called a Live View), has become popular.
In a camera with such a Live View, an image of a subject can be displayed on a display panel, and a user can confirm an image of a subject and framing without viewing through a finder.
Therefore, in a camera with such a Live View, an image of a subject and framing can easily be confirmed even in photo shooting by setting a camera at unusual positions, for example, by placing a camera at a high position or a position close to the ground.
Further, in a camera with such a Live View, an image of a subject can be displayed as an electronic image, and various information such as “black defects” and “white blur” can be superimposed on the Live View.
As a focus adjustment method, a single-lens reflex camera generally employs a manual focus mode (MF), in which a focus is manually adjusted, or an autofocus mode (AF), in which a focus is automatically adjusted by detecting displacements of focus by a AF sensor provided in a camera.
In a single-lens reflex camera with a Live View, a AF sensor cannot be used while a Live View is being executed. Therefore, various methods are used for adjusting a focus while a Live View is being executed.
For example, in one method, a Live View is once stopped, and a picture is taken after a focus is automatically adjusted by using a AF sensor. In another method, a field of view is partially magnified to facilitate visual confirmation of a focused state, and a focus is manually adjusted.
A single-lens reflex camera has adopted “a Live View Autofocus”, which is a focus adjustment method widely used in a compact digital camera. Generally, a single-lens reflex camera has a longer focal distance than in a compact digital camera. Thus, a depth of field is shallow, and a shooting field is largely different at infinity and close end.
Therefore, a subject with high contrast crosses a contrast detection area, and a false peak may be detected. As a result, a subject may be determined to be in-focus at an unintended focal position.
As a technique to prevent detection of a false peak (a false focus), Jpn. Pat. Appln. KOKAI Publication No. 2005-208274 discloses the following technique.
In the technique disclosed in the Publication No. 2005-208274, by moving a focusing lens from a close end to infinity, an integrated value (a focus evaluation value) by a video signal, which is obtained by eliminating a low-frequency component from an imaging signal, is calculated for a plurality of lens position, and a focus evaluation history is obtained in a first focus area, and in a second focus area for determining truth or false. A second focus area includes the whole first focus area, and is larger than the first focus area.
Thereby, a camera determines whether maximum positions of focus evaluation history in the first and second focus areas coincide, and calculates a focused lens position when the maximum values coincide, and does not calculate when the maximum values do not coincide, thereby preventing false focusing.